1. Field of the Invention
The present invention relates to an image display apparatus and, more particularly, to a compact head- or face-mounted image display apparatus which is retained on the user's head or face to project an image into his or her eyeball.
2. Background of Related Art
In recent years, helmet- and goggle-type head- or face-mounted image display apparatuses, which are designed to be retained on the user's head or face, have been developed for the purpose of enabling the user to enjoy virtual reality or a wide-screen personally.
For example, Japanese Patent Application Laid-Open (KOKAI) No. 3-191389 (1991) discloses an optical system including, as shown in FIG. 39, a two-dimensional display device 1 for displaying an image, a concave mirror 2 provided to face the display device 1 to project the displayed image into an observer's eyeball 4 as an enlarged image, and a half-mirror 3 disposed between the display device 1 and the concave mirror 2, thereby enabling the optical system to be arranged in a compact structure while maintaining favorable image forming performance. In U.S. Pat. No. 4,269,476, as shown in FIG. 40, a beam splitter prism 5 having a half-mirror is used in place of the above-described half-mirror.
There are two methods of widening the field of view of conventional ocular optical systems such as those shown in FIGS. 39 and 40: 1 one in which the size of the two-dimensional display device 1 is increased; and 2 the other in which the focal length f of the optical system is reduced.
With the method 1, however, not only the two-dimensional display device but also the half-mirror or the prism increase in size. Accordingly, the apparatus increases in both overall size and weight, which is unfavorable for a head-mounted image display apparatus. Further, when the apparatus is designed for observation with both eyes, the achievement of a wider field of view is limited because of interference between two optical systems for both eyes, which are composed of the above-described elements.
When the method 2 is employed to widen the field of view of an optical system arranged as shown in FIG. 39, the field curvature (negative Petzval sum) and positive comatic aberration produced by the concave mirror increase, giving rise to a problem. To solve the problem, if a Mangin mirror is used in place of the concave mirror so as to increase the capability of correcting aberrations such as field curvature and coma and to improve the durability of the mirror, chromatic aberration is produced by the refracting lens.
If a doublet 8 is used as a refracting lens, as shown in FIG. 41, chromatic aberration can be corrected, but the overall weight of the apparatus increases, which is unfavorable for a head-mounted image display apparatus. That is, to correct chromatic aberration effectively, the doublet 8 is composed, for example, of a concave lens made of a flint glass material, and a convex lens made of a crown glass material. Therefore, the weight of the constituent lens elements, particularly the weight of the concave lens, is heavy.
The method 2 further involves the following problem: If the focal length of the optical system is reduced with the size of the two-dimensional display device maintained at a constant level, the spacing between the eyeball 4 and the half-mirror 3 (i.e., the working distance of the optical system) reduces, so that it becomes difficult for the user to use the apparatus with his or her glasses on. Further, there is a limit to the achievement of a wider field of view.
One approach to widen the field of view without increasing the size of the optical system to a considerable extent is to dispose a surface having a positive refractive power or a positive lens between the eyeball and the half-mirror. In this case, however, it is necessary to correct chromatic aberration produced by the surface having a positive refractive power or the positive lens, which is disposed between the eyeball and the half-mirror.
FIG. 42 illustrates an optical ray trace of one example of the above-described optical system. The illustrated optical system uses a prism 5 to increase the spacing between the pupil E of the eyeball 4 and the half-mirror 3 and employs an aspherical surface as a surface 7 of the prism 5 on the side thereof which is closer to the image display device 1 in order to correct distortion, which increases as the field of view is widened, and to ensure the required telecentric properties.
The optical system having the above-described arrangement provides a wide field of view, i.e., 40.degree..times.30.degree., and yet it is compact and exhibits favorable performance. However, since the doublet 8 composed of glass lenses is provided in the vicinity of the reflecting mirror in order to correct chromatic aberration, the overall weight of the apparatus increases to a considerable extent, as described above, even if the prism 5 is made of a plastic material to suppress the increase of the weight.
In any case, if it is intended to form an achromatic optical system using glass lenses, the weight increases considerably, as described above. Therefore, it is very important to reduce the weight of the apparatus.